A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective NA of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein.
However, submersing the substrate or substrate and substrate table in a bath of liquid (see for example U.S. Pat. No. 4,509,852, hereby incorporated in its entirety by reference) means that there is a large body of liquid that must be accelerated during a scanning exposure. This may require additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects.
One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid supply system (the substrate generally has a larger surface area than the final element of the projection system). One way which has been proposed to arrange for this is disclosed in WO 99/49504, hereby incorporated in its entirety by reference. As illustrated in FIGS. 5 and 6 of the accompanying drawings, liquid is supplied by at least one inlet IN onto the substrate, preferably along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side. FIG. 5 shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of FIG. 5 the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of in- and out-lets positioned around the final element are possible, one example is illustrated in FIG. 6 in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element.
In a conventional lithographic apparatus, the substrate is often clamped to a burl plate (sometimes referred to as a pimple plate or table) by pressure differential between the atmosphere above the substrate and a partially evacuated space below the substrate. The burl plate has a plurality of projections or raised portions (hereafter referred to as burls), such as pimples or concentric rings, distributed over the space within a vacuum wall corresponding to the perimeter of the substrate. The term “burls” sometimes is considered to refer to a swirl configuration but must be considered herein in a broader context to refer to any orientation of projections or raised portions. The substrate rests on the burls and optionally the vacuum wall, which may be made lower than the burls so that there is a controlled leak of air into the space under the substrate, as described, for example, in U.S. Pat. No. 6,232,615, which is hereby incorporated in its entirety by reference. A principal advantage of a burl plate is that the total area of the tops of the burls is very small compared to the area of the substrate so that there is a correspondingly small chance that a contaminant particle on the backside of the substrate will come between the substrate and a burl and hence distort the substrate. Thus, burls may also be used when the clamping force is electrostatic rather than by pressure differential.